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Old 12-20-2012, 08:54 PM   #11
Bus Nut
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Join Date: Nov 2003
Location: Twin Falls, Idaho
Posts: 808
Rear axle numbers

Rear axle ratios are easy to understand. A high number means a low ratio, as 6:25 to 1, as this is a very low ratio.
School buses usually are ordered with a 5:60 to a 4:10. These are gears and speeds for a very controlled school route.
A low number is a high ratio, as 3:73, or 3:55 to 1. Many school bus conversions usually change gears for better miles per gallon and to reduce engine speed. Selecting a low number rear gears, will require some planning as usually the
number will require more power. Too high a rear ratio will make a huge load for a low power engine. A compromise is to install an overdrive unit between the transmission and rear axle. Frank
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Old 12-22-2012, 09:01 AM   #12
Join Date: Nov 2008
Location: Long Island NY
Posts: 148
Re: Had the diff swap done...

Originally Posted by thrash
Ok, sorry for the late follow up on this.

We travelled from ND to California and Back, going through Yosemitie, the Rockies, etc.

Average MPG on the trip (which was many thousand miles over 3 weeks): 9.8

I was _floored_ at how good the mileage was.

We had as low as 6 and as high at 14, depending on the driving segment.
That is fantastic mpg's! Glad to hear it does have an affect. Do you know what the average MPH was? 65? 75? 55? I once drove a GASOLINE RV (ford 460CI) across country at 52mph, BOTH WAYS, in order to save gas. I still only averaged 8-9 due to running the generator. It is still better then the 6MPG on previous trips when I did the speed limits! At 75mph, it was like throwing 50$ bills out the window every 50 miles.
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Old 12-22-2012, 07:32 PM   #13
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Join Date: Mar 2011
Location: Oregon/Philippines
Posts: 1,660
Re: Had the diff swap done...

Not all busses are geared for top speed of 58 or so. Mine was bought to take the teams to events, hence the compartments and over 70 mph top speed. My bus can average 10mpg if i am not pulling a trailer.
Jesus Christ... Conversion in progress.
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Old 12-25-2012, 06:32 PM   #14
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Join Date: Oct 2004
Location: New England
Posts: 1,009
Year: 1993
Coachwork: Ward Genesis
Chassis: International
Engine: DT466/MT643
Rated Cap: 77
Re: Had the diff swap done...

Same here: I think I have 4.11 gears...with 11R22.5 tires and a lockup Allison, 60MPH is only 1900RPM.
Optimism is a mental disorder.
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Old 12-27-2012, 07:23 PM   #15
Join Date: Sep 2011
Location: North Dakota
Posts: 26
Year: 1986
Coachwork: Thomas
Chassis: International S1800
Engine: DT466
Rated Cap: 77
Re: Had the diff swap done...

Prior to swap:

Vmax was 70mph (GPS confirmed), 3000 rpm indicated, in 5th gear, yielding 6.6 MPG

After swap:

Vmax is 90mph (GPS confirmed), i beleive at 3000 rpm indicated, still in 5th gear.

We never intentionally lowered our cruising speed to save fuel. Sometimes the terrain didn't allow us to run as fast as we wanted, in which case we were running the bus at WOT to maintain what speeds we could.

When the terrain/winds weren't limiting our speeds, we were generally running at posted limit +10mph, except when that would have exceeded 75mph, which IIRC is the rated speed of the tires. We would exceed 75 for a few minutes at a time (for instance, to verify the Vmax) when the terrain allowed, but generally our cruising speed was between 70 and 75.
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Old 05-21-2013, 10:08 PM   #16
Almost There
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Join Date: Feb 2009
Location: Columbus, Indiana
Posts: 79
Year: 1991
Coachwork: Thomas
Chassis: International
Engine: DTA 360, Fuller 6-speed
Rated Cap: 53
Re: Had the diff swap done...

I swapped my rear diff as well. I went from a 4.78 (69mph at fuel-stop 2,900rpm) to a 3.73 (88mph at fuel-stop). I haven't driven it 88mph, which there is no way it has the power to do, but it makes 75 no problem. The best thing is, with a 6-speed, I can now use 1st gear and don't shift into 6th until almost 65mph, and have a gear to go down into on the hills. Before, you couldn't downshift until you got below 50mph, which really, really, sux. I now cruise at 68mph and it's a lot quieter. before, I couldn't stand it past 60mph. I can roll now.

I bought a used drop-out section from a truck salvage yard for $800 and changed the pinion seal. Lower numerical gears aren't common in these axles--mine is an N175. Took a lot of calling and I had to drive 3 hours each way.

To handle the diff, I made a mount for it that bolted to the diff and welded the mount to my floor jack (it's a big 100# jack, not a little one--you need wide wheels for stability) If you do it where the bus is parked, and right on the old diff, the new one will line right up and go back in. That phawker is at least 300#, maybe #350.
My bus conversion
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